A surprising number of people dislike reading plain text emails so much that they want to convert all incoming mails to HTML. They don't care that it's bad netiquette to change the format of the reply unless absolutely necessary, such as to highlight or format text for easier reading or to embed pictures - they only want HTML.

While you can't readily change the format of all incoming email to HTML, you can change the fonts used to display plain text email so that's easier to read.

Open Tools, Options, Mail Format, Fonts dialog and change the font used for reading and replying to plain text messages.

Notes: When a sender uses plain text, you should always honor their choice - anyone who uses a cellular or wireless device to check email may not be able to read the HTML or they may be charged by the minute or KB to download messages (HTML messages are much larger than plain text).

Even when HTML formatting options would make it easier to get your point across, you should avoid changing the format if at all possible.